The terminology ("fructo-oligosaccharide") as used herein means oligosaccharides represented by formula (I) shown below, in which at least two fructose residues are bonded through a .beta.-1,2-linkage to form a main chain. ##STR1## wherein R represents a hydrogen atom or a glucose residue; and n represents an integer of from 1 to 5.
Fructo-oligosaccharides of formula (I) wherein R is a hydrogen atom include inulobiose, inulotriose, inulotetraose, inulopentaose, etc. These fructo-oligosaccharides are known to be naturally present in artichokes, cluster-amaryllis (Lycoris radiata Herb.), etc. and can be prepared by hydrolysis of inulin or lycoricine with an acid or an enzyme.
The fructo-oligosaccharides of formula (I) wherein R is a glucose residue include 1-kestose, nystose, fructosylnystose, etc. These oligosaccharides are also known to be present in artichokes, onions, leeks (Allium odorum), etc. They can be synthesized by fructose inversion of sucrose by the action of invertase. Industrial production of fructo-oligosaccharides by utilizing the action of invertase is known in the art.
Although fructo-oligosaccharides are less sweet than sucrose, they have excellent characteristics which are not associated with sucrose. One of the characteristics is their excellent effect on the growth of intestinal flora, particularly Bifidobacterium. That is, fructo-oligosaccharides ingested are not susceptible to decomposition by human or animal digestive enzymes and easily reach the large intensine, where they are selectively utilized by Bifidobacterium to accelerate the growth of Bifidobacterium. Studies have demonstrated that keeping the number of Bifidobacterium above a certain level in the intestine is important for health maintenance. In fact, it has been reported that Bifidobacterium increased upon ingestion of fructo-oligosaccharides brings about constipation or diarrhea relief and a reduction of intestinal putrefaction caused by noxious bacteria. Therefore, when fructo-oligosaccharides are given to animals, they contribute to the promotion of growth and improvement of feed efficiency. Further, administration of fructo-oligosaccharides produces physiological effects, such as improvement in lipemia, reduction in cholesterol level in the serum, reduction in blood pressure, and the like. These effects are considered to arise due to the actions upon the intestinal flora.
In addition, fructo-oligosaccharides have been reported to hardly cause dental caries. Dental caries is attributed to Streptococcus mutans, a lactic acid bacteria in the oral cavity. The mechanism of the development of dental caries is as follows. Streptococcus mutans secretes dextran synthetase which acts on sucrose, a substrate, to synthesize dextran. The dextran sticks to the surfaces of teeth to form dental plaques thereon, in which Streptococcus mutans ferments fermentable saccharides anaerobically to produce lactic acid thereby to cause decalcification of the dental enamel. When fructo-oligosaccharides are involved in this mechanism, they do not serve as substrates either for dextran synthesis due to dextran synthetase secreted from Streptococcus mutans or for anaerobical fermentation of Streptococcus mutans. The fact that the fructo-oligosaccharides hardly cause dental caries has been demonstrated in animal experiments.
As described above, fructo-oligosaccharides are very useful, and establishment or improvement of an industrial process for preparing them is considered greatly worthwhile.
It is possible to produce fructo-oligosaccharides from inulin or a plant containing inulin by hydrolysis using acids or enzymes as mentioned above. Inulin is relatively labile to acids, and its hydrolysis is generally carried out in the laboratory using oxalic acid. However, oxalic acid is not favorable for use in industrial production of food materials because it is highly toxic and low yields are attained therewith. As a result, a process comprising hydrolyzing inulin by the action of inulase has been employed. Known inulase includes those of plant origin and those of microorganism origin. In general, the former has low activity, and the latter has high exo activity that is suitable for the production of fructose but unsuitable for the production of oligosaccharides. Therefore, production of inulase of high endo activity suitable for the production of oligosaccharides requires removal of inulase of high exo activity from a culture or powder of a microorganism containing both types of inulase and fractionation and recovery of inulase having high endo activity.
Conventionally known processes for obtaining inulase having high endo activity include adsorption and desorption by column chromatography using organic ion-exchangers, e.g., DEAE-Sephadex A-50; fractionation and purification by gel-filtration column chromatography using Sephadex G-50, etc.; and combinations thereof. These techniques are not suitable, however, for obtaining inulase for use in the mass production of foods at low cost, although they are excellent when applied for the purpose of obtaining highly purified inulase for use in, for example, laboratories. Hence, it has been desired to develop a technique for preparing inulase of high endo activity on an industrial scale.